In-mold foamed articles prepared from polypropylene resin pre-expanded particles have advantages such as the capability to be formed into desired shapes, lightweight, and thermal insulation ability. When compared with articles prepared by using a similar type of pre-expanded particles, the in-mold foamed articles prepared from polypropylene resin pre-expanded particles have chemical resistance, thermal resistance, and distortion restoration rate after compression superior to those of in-mold foamed articles prepared from polystyrene resin pre-expanded particles, and exhibit higher dimensional accuracy, thermal resistance, and compression strength than those of in-mold foamed articles prepared from polyethylene resin pre-expanded particles. The in-mold foamed articles prepared from polypropylene resin pre-expanded particles having these features thus are applied for various uses including automobile interior components, automobile bumper core materials, thermal insulation materials, and cushioning packaging materials.
Molding machines used today for the production of in-mold foamed articles from polypropylene resin pre-expanded particles are mostly of a type that withstands a pressure of 0.4 MPa, and the pressure of the heating steam for molding usually employed for the molding machines is up to about 0.36 MPa. The polypropylene resin pre-expanded particles used for in-mold foaming are composed of a resin having properties that comply with these conditions. Typically, an ethylene-polypropylene random copolymer having a melting point of about 140° C. to 150° C. is used.
The uses of the in-mold foamed articles include many that require high rigidity, such as automobile interior components and automobile bumper core materials. The rigidity of the in-mold foamed articles generally depends on the rigidity of starting material resins and expansion ratios. The in-mold foamed articles applied for such uses currently achieve the required high rigidity by using a product having a high density, i.e., a product foamed at a low expansion ratio. However, the increase in density degrades the lightweight originally expected from the in-mold foamed articles. In particular, this results in a decrease in fuel consumption of automobiles equipped with components composed of such articles and also in an increase in weight of the ultimate waste product. Thus, an increase in density should be avoided where possible. On the other hand, by increasing the rigidity of the resin in itself, which is another possible approach for achieving high rigidity, the manufacturing conditions for in-mold foaming become more stringent, and this increases the cost for the foaming process. In detail, a polypropylene resin having high rigidity usually has a low comonomer content and a high melting point. The pressure of the heating steam for molding required to obtain a satisfactory molded article tends to increase with the increase in melting point of the resin. Thus, in order to achieve higher rigidity, a molding machine and a die that can withstand high pressures must be used, thereby increasing the equipment cost and utility cost. As a result, the cost for the molding process is increased.
In recent years, the importance of the appearance is increasing even for the in-mold foamed articles. The appearance is notably important for uses, such as automobile interior components and returnable boxes, that catch people's eyes. The in-mold foamed articles are required to have good appearance in addition to physical properties, such as rigidity, lightweight, and thermal insulation ability, usually required for the in-mold foamed articles. Owing to their production process, the in-mold foamed articles have inter-particle gaps and honeycomb patterns due to particle shapes in their appearance. Many products that require good appearance cannot accept such appearances. In order to render inter-particle gaps less noticeable, for example, the pressure of heating steam during the in-mold foaming generally is increased to promote the fusion among the particles. In order to remove the honeycomb patterns derived from the particle shape, a technology that uses a die having surface provided with fine irregularities (refer to, for example, Patent Document 1 shown below) has been practiced. According to this technology, the pressure of the heating steam during the in-mold foaming is set to a high level to promote the transfer of the irregularities onto the foamed articles. As apparent from these technologies, the pressure of the heating steam for molding during the in-mold foaming must be increased to a level higher than that required for fusion of the particles in order to obtain good-appearance in-mold foamed articles whose inter-particle gaps are hardly noticeable, i.e., in order to obtain in-mold foamed articles with good surface appearance.
Thus, a technology that stably can produce high-rigidity polypropylene resin in-mold foamed articles with good surface appearance at a low molding processing temperature without using a special molding machine is desired.
Various technologies for improving the rigidity of the in-mold foamed articles are now being investigated. A conceivable approach simply is to use homopolypropylene to obtain a high rigidity with a polypropylene resin. For example, Patent Document 2 shown below discloses a technology related to homopolypropylene resin pre-expanded particles whose tensile modulus is 15,000 to 25,000 kg/cm2 and the heat of the high-temperature-side peak in a differential scanning calorimetry (DSC) curve observed with a differential scanning calorimeter is 30 to 60 J/g. Moreover, Patent Document 3 shown below discloses a technology capable of preparing pre-expanded particles that may produce an in-mold foamed article at a relatively low molding temperature using a homopropylene resin whose melt flow rate (MFR) is 20 to 100 g/10 min.
However, according to the technology disclosed in the Patent Document 2, the pressure of the heating steam during the molding required for obtaining a satisfactory in-mold foamed article is described as being in the range of 0.4 to 0.6 MPa. Thus, a molding machine that withstands a pressure of only up to 0.4 MPa may not be used for molding. Moreover, the surface appearance of the resulting article is not particularly described. According to the technology disclosed in the Patent Document 3, although homopolypropylene and a random polypropylene resin with a low comonomer content are used, no specific description is provided regarding the surface appearance. This technology also sets a similar evaluation standard, i.e., whether the fusion between the expanded particles is observed at a ratio of 60% or more. However, this standard is for evaluating whether local fusion between particles inside the in-mold foamed article occurs or not. Unlike the standard for obtaining surface appearance, such a standard is satisfied easily by use of low molding heating steam pressure. It is presumably difficult practically to obtain a molded article with good surface appearance using a molding machine that withstands a pressure of up to 0.4 MPa according to the technology disclosed in this related art document.
A technology that uses polypropylene random copolymers is also being investigated from the standpoint of high moldability, although the rigidity of the resulting article is not as high as that achieved by homopolypropylene. For example, Patent Document 4 shown below discloses a technology for obtaining a foamed article having a high compression strength and moldability by using a resin whose ratio of the weight-average molecular weight Mw to the number-average molecular weight Mn is 6 or less and whose ratio l/lo of the diameter l of the resin discharged from an orifice to the diameter lo of the orifice is 1.15 or less when the MFR is measured with a MFR meter according to Japanese Industrial Standard (JIS) K-7210. However, this technology is merely a technique for realizing the production of a satisfactory in-mold foamed article without impregnating pre-expanded particles with inorganic gas or without filling a die with pre-expanded particles in the compressed state in the course of in-mold foaming. The effect of enhancing the compression strength of the in-mold foamed article rarely is exhibited according to this technology. In fact, the compression strength of the foamed article described in this document is at most 2.9 kgf/cm2 at a molding heating pressure of 3.0 kgf/cm2. Such a compression strength is not significantly different from that of polypropylene resin foamed articles commonly used today. According to this technology, a polypropylene resin foamed article having a compression strength of 3.3 kgf/cm2 barely is obtained at a high molding heating pressure of 3.5 kgf/cm2. Furthermore, although the inter-particle gaps of the molded articles are evaluated, it is not likely that the good surface appearance is realized with this technology.
Moreover, Patent Document 5 shown below discloses a technology that uses resin particles having a melting point of 155° C. to 165° C., a ratio Mz/Mw of the Z-average molecular weight to the weight-average molecular weight of 3 to 6, and a MFR of 10 to 150 g/L. The main objective of this technology is to obtain expanded particles for in-mold foaming without using a so-called “DOKAN” method. Moreover, as evident from the fact that the melting point of the resin exceeds 155° C., the heating condition for obtaining a foamed article is as high as a level exceeding 4 kgf/cm2.
Patent Document 6 shown below discloses a technology that uses a base resin composed of a propylene random copolymer having a melting point of 149° C. to 157° C., a MFR of 1 to 20 g/10 min, and a half crystallization time not exceeding a certain value.
Moreover, Patent Document 7 shown below discloses a technology for increasing the compression strength of an in-mold foamed article obtained by setting the relationship between the amount of crystals at the high-temperature-side of a crystal fusion curve and that at the low-temperature-side within a particular range. Here, the crystal fusion curve is obtained by the observation of the crystals of polypropylene resin pre-expanded particles for in-mold foaming by differential scanning calorimetry (referred to as “DSC” hereinafter).
However, these technologies require heating steam having a pressure as high as 0.4 to 0.5 MPa for in-mold foaming and are practicable only by using a highly pressure resistant molding machine, as with the technologies described in the above mentioned Patent Document 2 and 3.
Furthermore, Patent Document 8 shown below discloses a technology for obtaining a resin having a high tensile modulus for the resin melting point, i.e., a high rigidity, by using a polypropylene resin containing 1-butene as a comonomer and for obtaining a high-rigidity in-mold foamed article from this resin.
This technology also requires heating steam having a pressure of about 0.4 MPa for in-mold foaming. Although the pressure of the molding heating steam is relatively low compared to other technologies, the lowest pressure in the practical examples is 0.36 MPa, which is very close to the upper limit of the pressure, 0.4 MPa, employed in common molding machines. Moreover, no specific description is provided for surface appearance. It is considered that a higher molding heating steam pressure is necessary to obtain a good surface appearance.
Furthermore, Patent Document 9 shown below discloses a technology for obtaining a high-rigidity polypropylene resin foamed article by using polypropylene resin pre-expanded particles whose base resin is a propylene/1-butene random copolymer containing 3 to 12 percent by weight of 1-butene. According to the description, this technology permits using a common molding machine that withstands a pressure of up to 0.4 MPa since the pressure of the molding heating steam is about 0.3 MPa. According to an example described in this document, however, the rigidity of the in-mold foamed article obtained by a molding heating steam pressure of or near 0.3 MPa is 6.2 kg/cm2 of compressive strength measured according to Japanese Industrial Standard (JIS) K-6767 under a 50% compression strain at 20° C. This level of rigidity is not enough for uses that require high rigidity. Moreover, 1-butene single-system polypropylene resin random copolymer containing no ethylene component is hard and brittle compared to ethylene-containing polypropylene resin random copolymers. Thus, when it is used as the base resin of the foamed article, the foamed article will have poor dimensional recovery ability after compression and low impact properties in a low-temperature region. Polypropylene resin foamed articles have lower rigidity but superior resistance to repeated impacts and flexibility when compared with polystyrene resin in-mold foamed articles. For these properties, the polypropylene resin also is used for cushioning packaging materials. Accordingly, the technology described in this document has a drawback in that the technology is not suitable for typical cushioning packaging uses other than those that require only high rigidity.
As discussed above, a special molding machine that can withstand high molding heating steam pressure has been used for applications that require high rigidity. However, in order to increase the pressure resistance of the molding machine, the size of the machine must be increased to increase the strength of the molding machine. Moreover, the thickness of a die also must be increased. These factors substantially increase the equipment cost, which is problem.
Furthermore, increasing the pressure of the molding heating steam leads to an increase in the amount of steam required for heating during molding. Thus, the amount of cooling water must be increased, thereby also increasing the energy cost. Since the heating to a higher temperature is required, the heating for molding takes a longer time, and the process of cooling the heated die with cooing water also takes a longer time. Thus, the production cycle per product takes a longer time, thereby leading to a decreased production efficiency. Moreover, since the shape of the die for in-mold foaming is complicated, local concentration of stress may occur on the die during the molding and heating depending on the shape, which may lead to breaking of the die and a further increase in cost.
As is described above, increasing the molding heating steam pressure during the in-mold foaming generates various problems. The pressure of the molding heating steam is preferably as low as possible. According to the existing technologies, it is difficult to obtain high-rigidity polypropylene resin pre-expanded particles for in-mold foaming that stably can produce molded articles using a common molding machine that withstands a pressure of up to 0.4 MPa. Moreover, no technology that achieves the required surface appearance for the in-mold foamed articles has been available so far.
On the other hand, a technology for imparting a new property to a resin by incorporating a resin having a different physical property has been developed. Patent Document 10 shown below discloses polypropylene resin pre-expanded particles composed of a mixed resin containing 90 to 10 percent by weight of a polypropylene resin having a MFR of 6 to 10 g/10 min and 10 to 90 percent by weight of a polypropylene resin having a MFR of 0.5 to 3 g/10 min, the mixed resin having a MFR of 2 to 5 g/10 min. The document describes that a molded article having good surface quality and fusibility and free of sink marks(deformation) can be obtained in a short molding time by using these pre-expanded particles. This document primarily describes effects related to the molding time and does not particularly refer to the rigidity. Moreover, although the skin mark of the molded article is evaluated, no specific description on surface appearance is provided.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-108134.
Patent Document 2: Japanese Unexamined Patent Application Publication No. 8-277340.
Patent Document 3: Japanese Unexamined Patent Application Publication No. 10-45938
Patent Document 4: Japanese Unexamined Patent Application Publication No. 3-152136.
Patent Document 5: Japanese Unexamined Patent Application Publication No. 10-306173.
Patent Document 6: Japanese Unexamined Patent Application Publication No. 10-316791.
Patent Document 7: Japanese Unexamined Patent Application Publication No. 11-156879.
Patent Document 8: Japanese Unexamined Patent Application Publication No. 7-258455.
Patent Document 9: Japanese Unexamined Patent Application Publication No. 1-242638.
Patent Document 10: Japanese Unexamined Patent Application Publication No. 2000-327825.